This invention relates to local area network technology, and more particularly to a contention resolution media access control protocol for a local area network using orthogonal frequency division multiplexing.
Digital Subscriber Line (DSL) communication systems use existing telephone lines for high speed data communications. A DSL system essentially encodes digital data as analog signals at very high data rates using special modems. One signaling method used to transmit such analog signals is orthogonal frequency division multiplexing (OFDM), in which analog encoded data bits are transmitted as complex tones in a plurality of distinct frequency bins.
A number of Digital Subscriber Line (DSL) systems have been proposed. For example, a version known as Asynmmetric Digital Subscriber Line (ADSL) provides a system that applies signals over a single twisted-wire pair that supports xe2x80x9cplain old telephone servicexe2x80x9d (POTS) and high-speed duplex (simultaneous two-way) and simplex (from a network to the customer installation) digital services. Part of the proposed standard for ADSL is set forth in the Draft Proposed Revision of ANSI T1.413-1995xe2x80x94Interface Between Networks and Customer Installationxe2x80x94Asymmetric Digital Subscriber Line (ADSL) Metallic Interface (Sep. 26, 1997), which is hereby incorporated by reference.
Local area networks (LANs) allow multiple users (or xe2x80x9cnodesxe2x80x9d) to utilize a shared media, such as a twisted-pair wiring, to transmit and receive digital information. The concept of OFDM signaling is so useful that attempts are being made to extend DSL technology to LAN communication systems.
In order to permit sharing of a LAN media, a media access control (MAC) protocol is commonly used to resolve xe2x80x9ccontentionxe2x80x9d between the users seeking to access the media simultaneously. A MAC layer protocol is basically responsible for sorting-out contention on a shared medium broadcast channel. The protocol maintains states, timers, methods, etc. for managing when and how nodes transmit on the shared medium. For conventional serial transmission wired media, such as those complying with the IEEE 802.3-XX and Ethernet standards, industry has widely accepted CSMA/CD (carrier sense multiaccess with collision detection) as a method of resolving contention. However, CSMA/CD is not particularly efficient under load.
For OFDM-based LANs, a variety of possible MAC protocols have been proposed. One such MAC protocol is described in U.S. patent application Ser. No. 09/003,844, entitled xe2x80x9cMethod and Protocol for a Medium Access Control Layer for Local Area Networks with Multiple-Priority Traffic,xe2x80x9d filed Jan. 7, 1998 and assigned to the assignee of the present invention. In this MAC protocol, each node waits for a certain period of silence, then transmits a single randomly selected tone as an intent-to-transmit signal. All nodes receive the combined intent-to-transmit signals of all other signals. Each node makes a determination as to whether it had transmitted the highest frequency, and if so, begins to transmit data. If two or more nodes had sent the same frequency as an intent-to-transmit signal, a collision will occur, indicated by the presence of garbled data on the medium. The colliding nodes stop transmitting data and repeat the process above until no collisions exist. A drawback of this protocol is that each transmitting node must xe2x80x9clistenxe2x80x9d to what it is transmitting to detect a collision, which requires data demodulation. Data demodulation requires a substantial expenditure of processing resources and complex processing. Further, if collisions occur, time is spent resolving contention for the media rather than transmitting data.
Another MAC protocol is described in U.S. patent application Ser. No. 09/087,624, entitled xe2x80x9cAdaptive Tree-based Contention Resolution Media Access Control Protocolxe2x80x9d, filed May 29, 1998 and assigned to the assignee of the present invention. This protocol uses constant tones in frequency bins to resolve contention between multiple transmitting nodes. All nodes are synchronized to transmit xe2x80x9ccontention tonesxe2x80x9d in a xe2x80x9ccontention frame.xe2x80x9d Each node is assigned a unique identification (ID) number, and this ID number is mapped to frequency bins by transmitting a tone in each bin corresponding to each 1-bit in the binary representation of the node""s ID number. The result of multiple nodes each transmitting its unique ID during a contention frame is a complex frequency signal which each participating node decodes. The nodes then participate in a xe2x80x9cresolution framexe2x80x9d in which all nodes again transmit tones in frequency bins. The resolution frame tones correspond only to tones in which the result of the contention frame indicated the presence of a tone where such node had 0-valued ID bits. All nodes receive the results of the resolution frame and record the results (xe2x80x9cresolution bitsxe2x80x9d) in a reasonably-balanced binary tree. If the resolution frame contained no tones in any frequency bin, only one node is contending and can transmit. Otherwise, the contending nodes utilize the resolution bits to continue with another cycle of contention and resolution frames. Each node follows a tree-traversal algorithm to determine the order of transmission among all contending nodes. The contention process continues until all contending nodes have transmitted their frames.
While this last protocol provides positive identification of idle media, and does not require data demodulation, it still involves some computational complexity and, if media contention occurs, time is spent resolving the contention rather than in transmitting data.
Accordingly, the inventors have determined that it would be useful to have a contention resolution MAC protocol that is simple to implement, does not require data demodulation, does not require collision detection to resolve contention, and minimizes the time spent in resolving contention. The present invention provides a method and system for achieving this end.
The invention includes a contention resolution media access control protocol for a local area network using orthogonal frequency division multiplexing (OFDM). More particularly, the invention includes a MAC protocol for an OFDM-based LAN that is believed to be more efficient, under load, than CSMA/CD, and takes advantage of OFDM frames available on the channel physical layer. Additionally, the preferred embodiment of the invention does not require collision detection to resolve contention.
Under the inventive MAC protocol, each LAN node is assigned a LAN-unique node identifier that is the index of a Fast-Fourier transform (FFT) frequency bin or other discrete tone in the free band. The node identifier may be assigned or automatically discovered by the system. The node identifier is played in each contention cycle by each node that has data to transmit. Each contending node can easily determine which nodes are contending for transfer of data over the media channel from the presence of node identifiers within distinct frequency bins. Contention for the media channel is resolved by each contending node being giving access to the media channel in accordance with a preset order in which contending nodes will transmit.
In one aspect of the invention, the inventive method and system includes a technique for resolving contention for access to a media channel among a plurality of nodes on a local area network using orthogonal frequency division multiplexing for transfers of data and control information in a plurality of frequency bins. More particularly, the invention includes assigning at least one frequency bin of the media channel as a unique node identifier to each node; synchronizing all of the nodes to a common starting point for participation in a contention cycle; transmitting from each node contending for access to the media channel a signal in at least one frequency bin corresponding to such nodes"" assigned node identifier: decoding in each contending node the node identifier from each other contending node; and granting successive access to the media channel for each contending node in an order determined by the node identifier for each such contending node.
The inventive MAC protocol is simple to implement, does not require data demodulation, does not require collision detection to resolve contention, and minimizes the time spent in resolving contention.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.